The known artificial orthopaedic devices—among which are the external or prosthetic devices, the orthoses, and the internal prostheses or articular prostheses—find a wide use in the medical as well as chirurgical fields.
In the articulations (here we refer to a generic articulation although, for the sake of clarity we will refer to the knee articulation) there are surfaces, so called articular surfaces, that come to contact with each other and have a relative motion of rolling and sliding, and together define a desired relative motion.
This relative motion is guaranteed by articular structures such as ligaments, muscles and cartilaginous structures whose aim is, in particular, to maintain the relative position of the articular surfaces.
When the articular surfaces are impaired because of various pathologies or injuries or are missing because of traumatic or chirurgical amputations, their functionality may be restored by resorting to internal or external prostheses while, when the natural articulation is still existing but with a limited functionality, orthopaedic tutors are used to help or restore the impaired or compromised functions.
Internal prostheses try to replicate the impaired natural articular surfaces by substituting them by cams and other intermediate elements (for instance menisci of plastic material) which replicate the natural movement of the natural articulation.
In particular, the original movements is partly replicated by relying upon the shape of the conjugated surfaces of the prosthesis, which are manufactured with a shape similar to that of the original surfaces, and partially by relying on the natural articular structures not sacrificed during the surgical intervention necessary to implant the prosthesis.
It should be noticed that during the implantation of an internal prosthesis, the most important articular structures, such as for instance the cruciate ligaments in the knee prosthesis, are frequently removed in order to allow the insertion of the prosthesis and consequently the optimal conditions for maintaining the relative position of articular surfaces are destroyed.
The conjugate surfaces that come to contact with each other during the motion have a strong relative sliding motion which causes high wear due to friction and high loads per unit of surface.
In order to reduce the negative effects of the sliding, an intermediate element with a low friction coefficient is often introduced between the articular surfaces.
The external prostheses, used to completely substitute the articulation, are generally complex mechanisms which try to replicate at the best the relative motion of the substituted articulation and are therefore mechanically complex and expensive.
Similarly the existing orthoses guide the relative motion of the main elements of the natural articulation only approximately and are relatively complex.
Summarizing, the known orthopaedic devices are capable to replicate the natural motion only approximately, have elements which exhibit a high sliding relative motion with consequent strong wear which reduces their life, are mechanically complex and are expensive.
Moreover, in general it is very difficult to maintain the correct relative position between the conjugate surfaces which are in contact with each other, therefore (especially for internal prostheses) their relative motion can be hardly controlled.